The present invention relates to a high-precision measuring method and apparatus for measuring the frequency of a cyclically-repeating electrical signal. The invention is particularly useful in, and is therefore described below with respect to, the high-precision measuring method and apparatus described in prior U.S. Pat. No. 6,621,278 or U.S. Pat. No. 7,080,554, the contents of which patent and published application are expressly incorporated herein by reference.
As brought out in U.S. Pat. No. 6,621,278, and published U.S. Pat. No. 7,080,554 many measuring techniques are known for measuring or monitoring distance, displacement, temperature, pressure, force, and other parameters or conditions, but such known techniques generally increase in cost and complexity according to the precision desired, and also generally have an upper limit as to the precision practically attainable by the technique. For example, the measurement of distance of meters or kilometers with a precision of microns or fractions of a micron is extremely expensive, if attainable at all. The same limitations apply with respect to measuring temperature, force, and many other parameters or conditions.
U.S. Pat. No. 6,621,278 describes an extremely high-precision method and apparatus for measuring a predetermined parameter, or for monitoring a predetermined condition, having a known relation to or influence on the transit time of movement of an energy wave through a medium. The described method broadly involves transmitting a cyclically-repeating wave of the energy through a transmission channel in the medium from a transmitter to a receiver at the opposite ends of the transmission channel; continuously changing the frequency of the transmission according to changes in the monitored condition while maintaining the number of waves in a loop including the transmission channel as a whole integer; and utilizing the changes in frequency of the transmission to provide a measurement of the parameter or an indication of the monitored condition.
According to a preferred embodiment described in both of the above-cited patent and patent applications, the frequency of the transmission is continuously changed according to changes in the monitored condition by detecting a predetermined fiducial point in each cyclically-repeating energy wave received by the receiver, and changing the frequency of the transmitter in accordance with the detected fiducial point of each received energy wave such that the number of energy waves in the loop of the transmission channel is a whole integer. A second embodiment is described in the above-cited published patent application wherein the above operation is performed by a phase-locked loop circuit having an input from the receiver and an output controlling the transmitter. In both described embodiments, the changes in frequency of the wave transmission are measured to provide a measurement of the predetermined parameter, or an indication of the predetermined condition.
A conventional way of measuring frequency of a cyclically-repeating electrical signal is to count the number of cycles over a predetermined time period, and divide the counted number of cycles by the predetermined time period. However, where the electrical signal may vary in phase, such as in the above-described applications, such a measurement would produce an error which may vary up to the period of one cycle. For example, when measuring electrical signals in the order of one MHz, the error may be up to one microsecond, which corresponds to a frequency error of about 1000 Hz.